1. Field of the Invention
The present invention relates to sound absorbing structures adapted to sound chambers, and in particular to vehicle components having sound absorbing properties.
The present application claims priority on Japanese Patent Application No. 2008-22558, Japanese Patent Application No. 2008-55367, Japanese Patent Application No. 2008-69794, Japanese Patent Application No. 2008-104965, Japanese Patent Application No. 2008-69795, Japanese Patent Application No. 2008-111481, Japanese Patent Application No. 2008-223442, Japanese Patent Application No. 2008-221316, and Japanese Patent Application No. 2008-219129, the contents of which are incorporated herein by reference in their entirety.
2. Description of the Related Art
Conventionally, various sound absorbing structures have been developed and disclosed in various documents such as Patent Document 1 and Non-Patent Document 1.                Patent Document 1: Japanese Unexamined Patent Application Publication No. 2006-11412        Non-Patent Document 1: “Architectural Acoustics and Noise Insulation Plans” written by Sho Kimura, Shokokusha Kabushiki Kaisha, Feb. 20, 1981, p.p. 150-151        
Patent Document 1 teaches a sound absorbing structure which absorbs sound by a plate-shaped or diaphragm-shaped vibration member and an air layer lying in the space behind the vibration member (hereinafter, referred to as a plate/diaphragm vibration sound absorbing structure). In the plate/diaphragm-vibration sound absorbing structure, a spring-mass system is composed of a mass of a vibration member and a spring component of an air layer. The spring-mass system has a resonance frequency f [Hz], which is expressed using an air density ρ0 [kg/m3], a sound speed c0 [m/s], a density ρ [kg/m3] of the vibration member, a thickness t [m] of the vibration member, and a thickness L [m] of the air layer in accordance with equation (1).
                    f        =                              1                          2              ⁢              π                                ⁢                                    {                                                                    ρ                    0                                    ⁢                                      c                    0                    2                                                                    ρ                  ⁢                                                                          ⁢                  t                  ⁢                                                                          ⁢                  L                                            }                                      1              /              2                                                          (        1        )            
When the vibration member of the plate/diaphragm-vibration sound absorbing structure has an elasticity so as to cause an elastic vibration, the property of a bending system is additionally introduced due to the elastic vibration. Non-Patent Document 1 teaches a sound absorbing structure based on architectural acoustics, wherein the resonance frequency of the plate/diaphragm-vibration sound absorbing structure is calculated using a first-side length a [m] of the vibration member having a rectangular shape, an second-side length b [m], a Young's modulus E [N/m2] of the vibration member, and a Poisson's ratio σ [−] of the vibration member, and integral numbers p and q in accordance with an equation (2) and is used for acoustic design.
                    f        =                              1                          2              ⁢              π                                ⁢                                    {                                                                                          ρ                      0                                        ⁢                                          c                      0                      2                                                                            ρ                    ⁢                                                                                  ⁢                    t                    ⁢                                                                                  ⁢                    L                                                  +                                                                            [                                                                                                    (                                                          p                              a                                                        )                                                    2                                                +                                                                              (                                                          q                              b                                                        )                                                    2                                                                    ]                                        2                                    ⁡                                      [                                                                                            π                          4                                                ⁢                        E                        ⁢                                                                                                  ⁢                                                  t                          3                                                                                            12                        ⁢                        ρ                        ⁢                                                                                                  ⁢                                                  t                          ⁡                                                      (                                                          1                              -                                                              σ                                2                                                                                      )                                                                                                                ]                                                              }                                      1              /              2                                                          (        2        )            
In equation (2), the term (ρ0c02/ρtL) of the spring-mass system is added to the term of the bending system (subsequent to the term of the spring-mass system); hence, the resonance frequency becomes higher than the resonance frequency of the spring-mass system, which in turn makes it difficult to reduce the peak frequency of sound absorption.
The relationship between the resonance frequency of the spring-mass system and the resonance frequency of the bending system due to elastic vibration caused by the elasticity of a plate has not been sufficiently resolved; hence, it is not possible to achieve high sound absorption in low frequencies in the plate/diaphragm-vibration sound absorbing structure.